1. Field of the Invention
The present disclosure relates to a system for separating a mixture of at least one gas and at least one liquid into a heavy fraction mainly containing liquid and a light fraction mainly containing gas. The disclosure also relates to an inlet device for pretreatment of a mixture of at least one gas and at least one liquid to be separated into a heavy fraction and a light fraction.
2. Description of the Related Art
In the oil and gas industry, separators are known for separating the supplied mixture of liquid (oil and/or water) and gas into a stream of substantially gas and a stream of substantially liquid. Numerous separators are known for separating such gas/liquid mixtures.
Known from WO 03/074156 A1 is a separator consisting of an upright column (upright vessel) provided with an inlet for the mixture to be separated and a first and second outlet for the discharge of, respectively, the separated heavy fraction and light fraction of the mixture. In the known separator, the incoming mixture is separated in three stages.
In the first separation stage, a first liquid/gas separation is carried out by a pretreatment unit connected to the inlet. The known pretreatment unit is a vane-type inlet device which is placed on the inlet stub of the separating vessel and which is provided with a number of curved blades which uniformly absorb the moment of the incoming gas-liquid flow. The blades then guide the gas-liquid flow laterally into the lower compartment of the separating vessel. As a result of this controlled inflow of the gas-liquid mixture, a first part of the liquid will already be separated, whereby the liquid load on the agglomerating unit and separator mounted downstream is considerably reduced.
In the second separation stage, the mixture is forced through a horizontal “demister” or “coalescer,” for instance, formed of a mesh of wires or “mesh pad,” provided between a lower part of the vessel and an upper part of the vessel. As mentioned above, during infeed, a part of the liquid is already separated from the mixture by the pretreatment device. The separated liquid accumulates at the bottom of the lower compartment. The remaining part of the gas/liquid mixture is then guided through the coalescer. The liquid droplets in the mixture for guiding through the wire mesh collide with the wires and grow therewith into a liquid layer. If the speed of the supplied gas/liquid mixture is sufficiently low, the liquid from the liquid layer will drop back under the influence of the force of gravity into the lower compartment and fall into the liquid already present there.
In the third separation stage, the mixture is guided through one or more cyclones arranged in the upper compartment downstream of the agglomerating unit for further separating the mixture into a substantially liquid-containing mixture part and a substantially gas-containing mixture part. The mixture entering the cyclones is set into a rotating movement, whereby a heavy fraction, in which a relatively large amount of liquid is present, is flung against the outer wall of the cyclone and is discharged via openings in the side wall, thereby providing a further separation of the heavy and light fraction.
Cyclones with very high separation efficiency are known, for instance, disclosed in EP 1 154 862 A, the content of which should be deemed as interpolated herein. Described herein is an installation wherein a number of boxes with highly efficient axial recycle cyclones are arranged in the upper compartment of the vessel.
Instead of the vane type inlet device mentioned earlier, the first stage of the separation process may be performed by one or more cyclone-type pretreatment devices. The gas/liquid mixture entering the inlet nozzle of the separation vessel is guided to a cyclone where the mixture is set in spin by use of a swirl element including one or more guiding vanes or due to use of a tangential inlet to the main cyclone body. The heavy fraction of the mixture is thrown to the cyclone body outer wall, while the light fraction is being concentrated in the center of the cyclone flow body. The gas escapes upwards through a passage provided inside the cyclone flow body.
In conventional designs, the bottom part of the inlet cyclone, i.e., the heavy fraction outlet of the inlet cyclone, is submerged in liquid. This is due to the fact that there is a required static head of liquid needed in order to prevent gas from breaking out through the bottom of the cyclone. In applications where the available static head of liquid is too short, these kind of cyclonic inlet devices could not be used. This is because, if gas breaks through at the bottom of the cyclone, there is a possibility that large amounts of liquid are lifted upwards as the gas enters the liquid surrounding the cyclone. This liquid may in turn overload any second stage separation unit installed downstream of the inlet cyclone.
If an inlet cyclone were to be installed that is not (partially) submerged in the liquid in the lower compartment, then the following problems may arise. In some circumstances, too much gas escapes out of the liquid outlet of the inlet cyclone. If too much gas escapes from this outlet, the gas may have enough velocity to also entrain liquid upwards after leaving the bottom part of the cyclone from the liquid outlet. Also, the amount of gas leaving the bottom liquid outlet may be so large that it will interfere with the liquid surface. This may cause liquid to be entrained from the liquid surface, which causes the separation efficiency to deteriorate.
Another problem is that gas from the interior of the separation vessel may be sucked into the inlet cyclone through the liquid outlet of the cyclone. This may happen since there is a low pressure zone inside the inlet cyclone due to the rotating fluids. If gas is sucked into the inlet cyclone through the liquid outlet, this will block the liquid from being discharged properly. The result is that the liquid has to follow the gas flow upward through the passage in the flow body and the gas outlet of the inlet cyclone. This is the worst scenario as the separation efficiency of liquid becomes practically zero.
The present invention is directed to an inlet device and a system wherein the above-identified problems may be solved or reduced. The inlet device and system may also exhibit improved separation characteristics.